The safety factor of landslide is used as the important index predicting its long term stability and determining the control device. The effect of the groundwater level and the change of load by embankment or cutting on the safety factor of its landslide are treated as the important factors.
But, the effect of load change on the relation between the groundwater level and the safety factor is not yet discussed. How the critical level of groundwater, in which the safety factor is 1, changes according to the change of load is discussed.
As the result, it is pointed out that the embankment or cutting for the purpose of increasing the safety factor inversely happen to make decrease the safety factor according to the groundwater level. Moreover, it is pointed that the embankment or cutting for the purpose of increasing the safety for slowing down the creeping velocity happen to guide the slope to dangerous side by decreasing the critical level of groundwater.
Replacement of embankment or cutting by accumulation or melting of snow is the same result. It is necessarry for us to consider not only the safety factor but also critical level of groundwater in controlling the landslide.

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It is one of the ultimate goals for researchers who investigate slope movements to establish the method for prediction of the occurrence time of a slope failure. At the present time Saito's method based on the characteristics of creep curves is used most often to predict it. This method, however, does not always give a correct prediction of the occurrence time. The method should be examined under various conditions. As the accuracy of this method may greatly depend on the types of landslide, a series of model tests were carried out to clarify the relationship between the types of the movement of soil mass and the test conditions and to know the propriety of Saito's method.

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A kinetic analysis of a landslide with a circular slip surface and numerical caluculations are attempted on the condition of a rotation of a rigid body. Then, the approximate equations of the landslide movement of a gentle slope are introduced from the analysis mentioned above. The relation between the sliding distance and the elapsed time is expressed by a sine function of time and these equations can be applied to the landslides of slopes with less than 15 degrees gradient.

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